1. Field of the Invention
The present invention relates to an electric double layer capacitor with activated carbon electrodes, and more particularly to an improved activated carbon electrode for an electric double layer capacitor.
2. Description of the Related Art
Electric double layer capacitors are suitable for supplying a large electric energy for driving a motor of an electric car or for realizing an energy recycle system, because the electric double layer capacitors are high in energy density and power density. In order to have further increased the energy density, it was proposed to increase a density of activated carbon in the electrodes of the electric double layer capacitor.
Japanese laid-open patent publication discloses a conventional electric double layer capacitor with electrodes which comprise a mixture of an activated carbon containing phenol resin and a sulfuric acid solution. The activated carbon containing phenol resin comprises a carbon paste with a density of about 0.6 g/cm3. The sulfuric acid solution contains 0.1–0.8% by weight of poly-4-vinylpyridine. The conventional electric double layer capacitor has the following disadvantages.
The conventional electric double layer capacitor is small in capacity for a unit volume and large in equivalent series resistance (ESR). The reason why the capacity is small would be that the electrode density is small, for example, about 0.6 g/cm3, and thus a filling amount of activated carbon for a unit volume is small, whereby a contact area to an electrolyte is small. The reason why the equivalent series resistance (ESR) is large would be that a contact resistance between activated carbons is high.
Japanese laid-open patent publication No. 5-82395 discloses another conventional electric double layer capacitor with polarization electrodes which comprise a sintered solid-state activated carbon impregnated with a diluted sulfuric acid. The electrodes have a specific surface area in the range of 2000–3200 m2/gr. The electrodes have a density in the range of 0.42–0.6 g/cm3.
It is advantageous that activated carbon particles are sintered in order to obtain a large reduction in the contact resistance between activated carbons. It is, however, undesirable that the electrode density is small, for example, in the range of 0.42–0.6 g/cm3, and thus the capacity for the unit volume is small.
In the above circumstances, the development of a novel electric double layer capacitor with improved activated carbon electrodes free from the above problems is desirable.